Today measuring of blood flow rates in the human body is typical performed by adding 133Xe to the tissue and afterwards measuring locally the decay of radioactive radiation, which experimentally has been correlated with the flow rate of the blood. This known method is referred to as the 133Xe-wash-out method as the decay of radiation is governed by transportation of the radioactive isotope by the blood, i.e. the radioactive isotope is said to be washed out from the tissue.
This method has some major disadvantages:                The 133Xe-wash-out method requires a frequent delivery of the radioactive isotope 133Xenon which involves a considerable expenditure, about 1000 Kr. (Danish crowns) every 14. day.        The apparatus for registration of 133Xenon wash-out has a price of about 60,000 Kr.        As 133Xenon is a radioactive isotope it exposes ionising radiation to the tissue in which it is deposited. This is undesirable and the 133Xe-wash-out method has for ethical—and perhaps also for medical—reasons not been used for pregnant women and child patients.        
An attempt to overcome the problems related to the 133Xe-wash-out method has been proposed by using heat instead of the radioactive isotope. In this method, which for instance is disclosed in M. Midttun, P. Sejrsen and M. Colding-Jørgensen, Heat-wash-out: A new method for measuring cutaneous blood flow rate in areas with and without arteriovenous anastomoses; Clinical Physiology (1996), 16, 259–274, heat is applied locally to the skin until a steady temperature of the tissue has established. After the steady temperature has been established, the heat supply is turned off where after the temperature decrease in time is recorded.
On the basis of the recorded temperature decrease in time the blood flow rate is determined. Correlation of the temperature decrease and the blood rate in the heat wash-out method is based on comparing the temperature decrease with results determined by the 133Xe-wash-out method.
A major disadvantage of the heat wash-out method is that the wash-out of heat is dependent on at least the temperature of the blood, tissue and the surroundings thereto. This implies that the flow rate of the blood—and in general the fluid to be measured—increases with increasing temperature and decreases with decreasing temperature.
By only applying heat to the tissue to be measured the flow rate detected by the temperature decrease does not correspond to the undisturbed flow rate, i.e. the flow rate of the blood at normal tissue temperature.
Another problem involved in measuring flow rate by heat wash-out is that the surface of the skin—or in general the wall member—where heat is applied and the surrounding area F has to be held at a temperature guaranteeing that heat at least during the heat wash-out phase is not drained from the skin (or the wall member) to the ambience in order to have a correct measure of the transportation of heat done by the blood (or the fluid).
Furthermore, the correlation between the flow rate and the temperature of the tissue and thereby the fluid to be measured has been focused on in the above mentioned reference (Midttun et al, 1996) using the 133Xenon-wash-out method in comparison with the heat wash-out method.
Probes for simultaneous measuring the ptcO2, ptcCO2 and the blood flow rate in the tissue beneath the probes are disclosed in the International Patent Application WO 83/01510 and in the European Patent no. EP 0 656 760. The probes disclosed therein both evaluate the flow rate of the blood by thermostating the surface of the tissue to a constant temperature and then evaluate the blood flow rate on the basis of the heat dissipated constantly in the probe during measuring and on the basis of an estimated arterial temperature or deep body temperature.
One major problem in connection with these prior art probes is the estimation of the deep body temperature which is not directly available. Another major problem in connection with the prior art probe is connected to the temperature dependency of the flow rate of blood. When only a one sided measuring technique is applied, i.e. only heat wash-out, the flow rate determined is not the flow rate of the blood being present during undisturbed conditions, i.e. when heat is not exchanged actively by the probe.
Further apparatuses are known from U.S. Pat. No. 4,859,078 which discloses an apparatus for the non-invasive measurement of thermal properties and perfusion rates of biomaterials, and from DE 33 09 093 that discloses an apparatus for measuring the blood flow in the skin.